An elliptic core optical fiber is known as a polarization maintaining optical fiber.
The elliptic core optical fiber was developed, since an ordinary single-mode optical fiber is likely to be disturbed in mode conversion and polarization state, being affected by feeble disturbances (e.g., temperature, pressure, vibration, etc.), and therefore is less likely to allow the propagation of light with its phase information and polarization state stably kept.
The propagation constant difference Δβ=βx−βy between intrinsic cross polarization modes of an ideal single-mode optical fiber is 0, but that of an actual optical fiber is not 0 since the core is not completely round and is non-uniformly strained.
Because of the natural birefringence, there is a problem that mode conversion is likely to occur between the polarization mode in x direction and the polarization mode in y direction that propagate at slightly different velocities in the optical fiber.
Therefore, the propagation constant difference Δβ between two cross polarization modes is made larger to render the mode conversion less likely to occur, for preventing the polarization state from being disturbed by feeble disturbances, thereby improving the properties of the single-mode optical fiber. This is a polarization maintaining optical fiber.
As a method of producing an elliptic core optical fiber used as such a polarization maintaining optical fiber, known is a method comprising the steps of grinding the periphery of a original preform having a sectionally circular core disposed at the center of a sectionally circular clad, along the parallel chords equally spaced from the central line of the original preform, to produce a processed preform, and drawing the processed preform with heating. (See JP, 6-235838, A.)
In this method, if a original preform is ground along the parallel chords equally spaced from the central line of the original preform and drawn with heating, an optical fiber can be spun from the tip of the molten preform.
In this case, in the molten portions, the portions ground in parallel of the clad are swollen relatively to the circular arc portions due to the surface tension acting on the surface of the optical fiber, making the optical fiber sectionally circular as a whole. Consequently, the circular core is deformed to be elliptic, and an elliptic core optical fiber is obtained.
The polarization maintaining optical fiber is not required to have specifically desired dimensions in the final form of the elliptic core, and to say extremely, it is only required that the core is formed to be elliptic, not circular.
Since it is only required that a polarization maintaining optical fiber is made larger in the propagation constant difference between intrinsic cross polarization modes, it is not necessary to keep the ratio of the minor axis to the major axis of the elliptic core at any specific value in the production of the optical fiber It is only required that optical fibers constant in the form of the elliptic core can be produced as a result of grinding under constant conditions, using the original preforms designed to be dimensionally constant.